1. Field of the Invention
The interest to use so-called high strength steels and fine-grained steels in the shipyard industries, in pressure vessels and machine constructions has increased very much during the years. Many of the metallurgical and welding technical problems and questions which have come up have been solved or are theoretically solved. For the moment the development work within the above-mentioned welding areas is focused partly to the question how to, through different welding methods and the choice of welding parameters, prevent that hard, brittle and crack sensitive zones appear in the heat affected zone as a consequence of heat generated during the process and partly to the development of filler metals which give weld deposits combining the wishes of high yield strengths and tensile strengths with high demands on the toughness. Especially the fulfillment of the specifications of the toughness demands at low temperatures causes great problems.
2. Description of the Prior Art
In steel production minor additions of elements as Al, Ti, Nb, V, B, Zr etc., which contribute to make a more fine-grained microstructure of the steel plate material and thus increase the yield and tensile strengths combined with improved toughness more and more have been used for the so-called fine-grained steels with hield strengths &gt; 36 kp/mm.sup.2 and tensile strengths up to 100 kp/mm.sup.2. Such elements make the steel plates more resistant to the influence of welding heat input -- slower to transform -- which from the weldability point of view is important. The necessary level of alloying elements earlier used in conventional steels in order to increase the strength such as carbon, Cr, Mo, Ni etc. has hereby been lowered, which has contributed to further improve the weldability. Considering the toughness qualities and the crack resistance of the weld deposit the limitation of the carbon content of the steel plates is in this case especially valuable as high carbon contents in the weld deposit may cause hot cracking and deteriorates the obtaining of good impact properties. In connection with this it should be pointed out that in most welding processes the dilution of base metal is at least 25%.
In spite of the above-mentioned improvements the use of the high strength steels however still causes a lot of welding problems, which demand necessary considerations to the realization of the welding not necessary when using conventional steels. By different heat treatments before and/or after welding satisfactory weld joints can certainly be achieved but practical, technical and, above all economical aspects prevent such solutions of the complex problems. The use of high strength steels is therefore still rather small.